U.S. patent number 4,790,052 [Application Number 06/877,254] was granted by the patent office on 1988-12-13 for process for manufacturing homogeneously needled three-dimensional structures of fibrous material.
This patent grant is currently assigned to Societe Europeenne De Propulsion. Invention is credited to Pierre Olry.
United States Patent |
4,790,052 |
Olry |
December 13, 1988 |
Process for manufacturing homogeneously needled three-dimensional
structures of fibrous material
Abstract
Method and apparatus for producing homogeneously needled
three-dimensional structures formed of superposed layers of fibrous
material and the needled structures produced thereby. As layers of
fibrous material are added to a stack of needled layers, the
needling needles are moved away from the stack so as to maintain a
uniform depth of needling over the entire stack, and to achieve a
uniformly needled result. To permit the same density of needling in
the initial layers, a substrate layer of needle penetrable material
is used to support the initial layers which can then be needled as
through they were layed on top of other needled layers. For the
final layers, the needling rate is reduced to compensate for the
lessened clogging of the needle in having fewer layers to penetrate
in the final needling. Needled structures according to the
invention find particular application in the manufacture of rocket
motor parts, heat protective pieces, and friction pieces such as
high performance brake discs or pads as used on aircraft and land
vehicles.
Inventors: |
Olry; Pierre (Bordeaux,
FR) |
Assignee: |
Societe Europeenne De
Propulsion (Surenes, FR)
|
Family
ID: |
27251189 |
Appl.
No.: |
06/877,254 |
Filed: |
June 23, 1986 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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685056 |
Dec 20, 1984 |
4621662 |
Nov 11, 1986 |
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Foreign Application Priority Data
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Jun 27, 1985 [FR] |
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85 09820 |
Jun 27, 1985 [FR] |
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85 09821 |
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Current U.S.
Class: |
28/110; 156/148;
28/114; 28/143 |
Current CPC
Class: |
B29C
33/56 (20130101); B29C 33/76 (20130101); B29C
53/8041 (20130101); B29C 70/24 (20130101); D04H
18/02 (20130101); F16D 69/02 (20130101) |
Current International
Class: |
B29C
33/76 (20060101); B29C 33/56 (20060101); B29C
53/80 (20060101); B29C 53/00 (20060101); B29C
70/24 (20060101); B29C 70/10 (20060101); D04H
18/00 (20060101); F16D 69/02 (20060101); D04H
001/46 (); D04H 003/10 (); D04H 005/02 () |
Field of
Search: |
;28/107,110,113,114,143
;156/148 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1660786 |
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Oct 1970 |
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DE |
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2324985 |
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Jan 1974 |
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DE |
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2434242 |
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Jan 1976 |
|
DE |
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1570992 |
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Jun 1969 |
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FR |
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2378888 |
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Aug 1978 |
|
FR |
|
2414574 |
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Aug 1979 |
|
FR |
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2506672 |
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Dec 1982 |
|
FR |
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931611 |
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Jul 1963 |
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GB |
|
1308999 |
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Mar 1973 |
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GB |
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1380518 |
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Jan 1975 |
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GB |
|
1549687 |
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Aug 1979 |
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GB |
|
2012671 |
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Aug 1979 |
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GB |
|
2048424 |
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Dec 1980 |
|
GB |
|
2099365 |
|
Dec 1982 |
|
GB |
|
Primary Examiner: Mackey; Robert K.
Attorney, Agent or Firm: Weingarten, Schurgin Gagnebin &
Hayes
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
The present application is a Continuation-in-part of U.S. patent
application Ser. No. 685,056 filed on Dec. 20, 1984, now U.S. Pat.
No. 4,621,662, issued on Nov. 11, 1986, commonly assigned and
incorporated herein by reference.
Claims
I claim:
1. A process for manufacturing homogeneously needled
three-dimensional structures of fibrous material to be used as
reinforcing structures in composite materials, said process
comprising the steps of:
superposing on a support layer after layer of fibrous material in
sheet form;
individually needling each layer with a substantially same and
constant needling density per unit of surface by thrusting needling
needles through said layer and to a predetermined depth below said
layer;
increasing the distance between the support and the needles by a
value substantially equal to the thickness of a needled layer each
time a new layer is superposed so as to keep said depth at a
substantially constant value with each newly superposed layer;
and
after a final layer has been superposed and needled, carrying out
additional finishing needling steps with the distance between the
support and the needles being increased after each finishing
needling step during which the structure is once needled over the
whole outer surface of the final layer so as to have a needling
density in the last superposed layers substantially equal to the
inside the other layers, whereby a substantially uniform needling
density is achieved through the structure.
2. A process as claimed in claim 1, wherein t he number of
finishing needling steps is less than would be necessary until the
needles cannot reach the final layer.
3. A process as claimed in claim 1, wherein said sheet of fibrous
material is formed by unidirectional webs which are superposed in
criss-cross fashion and preneedled.
4. A process as claimed in claim 3, wherein the different
unidirectional webs form between them angles of about 60
degrees.
5. A process as claimed in claim 3, wherein the webs are formed,
independently of each other, of fibers of a material selected from
the group consisting of carbon, ceramics, carbon precursors and
ceramics precursors.
6. A process as claimed in claim 1, wherein said sheet of fibrous
material comprises at least one fabric layer.
7. A process as claimed in claim 6, wherein said fabric is formed
of continuous filaments and is preneedled together with a card
web.
8. A process as claimed in claim 6, wherein said fabric is formed
of a discontinuous filaments and is preneedled together with a card
web.
9. A process as claimed in claim 1, wherein said sheet of fibrous
material comprises at least one fabric layer which is preneedled
together with a card web, said fabric and said card web being
formed, independently of each other, of a material selected from
the group consisting of carbon, ceramics, carbon precursors and
ceramics precursors.
10. A process as claimed in claim 9, wherein the warp and the weft
of the fabric are formed, independently from each other, of fibers
of a material selected from the group consisting of carbon,
ceramics, carbon precursors and ceramics precursors.
11. A process as claimed in claim 1, wherein support has a surface
coated with a base layer designed to allow the penetration of the
needles into the base layer without damaging the needles during the
needling of the first layers.
12. A process as claimed in claim 1, comprising the steps of
superposing flat sheets of said fibrous material on said support in
the form of a platen; individually needling each sheet over its
whole surface by moving the needles and the platen with respect to
each other in a direction perpendicular to the movement of the
needles during needling; and superposing a new sheet on the
preceding one at the end of a stroke in said direction.
13. A process as claimed in claim 1, wherein a continuous strip of
said fibrous material having a width substantially equal to the
axial dimension of an axisymmetrical structure to be manufactured
is wound on a support in the form of a mandrel so as to form
superposed layers.
Description
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to the formation of high uniformity
in needled structures particularly for use as reinforcing
structures in forming rocket motor parts, heat protective pieces
and high performance brake discs or pads as used on aircraft and
land vehicles.
Brake discs of the type presently used in jet aircraft and Formular
One racing must endure very high braking loads and the resulting
shear forces that are experienced in the brake disc material. In
the case of aircraft brakes, the thickness of the disc may be two
inches thus accentuating the shear on the disc material.
To withstand such shear forces, the discs must be manufactured to
great accuracy so that there is as much uniformity in the disc
material as is possible. A nonuniform disc material would create
local stress concentrations that will increase the possibility of
pad failure.
One method of manufacturing such brake disc in the past has used
the technology of needling together a plurality of layers of a
fibrous material which is then carbonized and densified by a matrix
material, for example by carrying out a chemical vapor deposition
process, to obtain a composite brake disc. The uniformity of the
resulting brake disc is a function of the uniformity of the
needling process. Prior needling processes have however suffered
from lack of uniformity resulting from the fact that in bonding
together a thick stack of many layers, the needling effects
differently the layers at the top than those at the bottom. As the
needles used in needling penetrate the layer of material, they pick
up and become clogged with the fibers of the fibrous material. As a
result, the needles become increasingly less effective in producing
the interlayer fiber crosslinking as they penetrate the stack of
layers. The result is a non-homogeneously needled stack. In the
critical applications of high performance brake discs, it is then
impossible to optimize the needling throughout the layers. If it is
optimized at the top of the stack, the needling variations
throughout the stack will produce suboptimal results at the bottom,
and vice-versa. It thus becomes difficult to realize the required
disc strength throughout its thickness.
With regard to the prior art, a known process for manufacturing
flat structures by needle superposed layers of fibrous fabric is
described in U.S. Pat. Nos. 3,971,669 and 3,994,762. According to
this known process, flat unidirectional layers are superposed in
criss-crossed fashion and then needled. Although no limitation is
given as to the number of superposed layers, this document gives no
indication as to the means that should be used for producing a
thick structure with homogeneous characteristics therethrough.
French Pat. No. 2,414,574 discloses a process for manufacturing
fibrous reinforcements for brake discs, by forming rings of felt by
needling, stacking the rings one over the other until the required
thickness is reached, and maintaining the resulting stack for
eventual densification. It is indicated that the stack of rings can
be needled, but no description is given as to what method is used
to this effect.
British Pat. No. 1,549,687 also discloses a process for
manufacturing fibrous reinforcements for brake discs. Annuli of a
fibrous material are plied together to form a stack which is passed
through a needle-punch loom. Further series of plies may be added
to the initially needled stack and the resulting stack is passed
through the needle-punch loom. The needle penetration can vary from
one needling pass to the other, but the result does not appear to
be an homogeneously needled structure. In addition, the thickness
of the stack is limited by the height of the loom. Compression of
the stack between needling passes, as suggested, will not help in
obtaining structures with uniform needling density and with
unlimited thickness.
Regarding now the manufacturing of cylindrical structures by
needling layers of fibrous fabric, British Pat. No. 2,099,365
describes a process which consists in winding a sheet of fibrous
fabric on a cylindrical mandrel and carrying out a needling
operation on the sheet while on the mandrel. This document,
however, gives no indication as to what means to use in order to
effectively produce a thick structure with a constant needling
density right through the thickness of the structure. The same
applies to French Pat. No. 2,378,888 and to U.S. Pat. No.
3,772,125, and to the prior art documents relating to the
production of tubular structures
thin walls (such as for example FR No. 1,570,992, GB No. 2,048,424
and U.S. Pat. No. 3,909,893).
Now it has been found that the technique of needling through small
thicknesses is not readily adaptable to large thicknesses. One
reason for this is that, once they have penetrated to a certain
thickness in the superposed layers, the needles loose their
effectiveness because their barbs become clogged up with pieces of
fibers torn from the layers of materials gone through by the
needles; the needles cannot thus fulfill their function correctly,
and as a result, the same needling characteristics cannot be
obtained throughout the whole stack.
Yet structures destined to be subjected to strong thermo-mechanical
stresses as in brake discs, it is important to keep the properties
constant throughout the structure, in order for example, to avoid
delamination.
It is the object of the present invention to propose a process
permitting the production of thick three-dimensional structures by
needling of superposed layers, with a constant density of needling
throughout the thickness of the structure, to allow the production
of homogeneous fibrous structures with a constant density of fibers
throughout the structures.
SUMMARY OF THE INVENTION
According to the teachings of the present invention, a process and
apparatus are provided for producing homogeneity of needling of a
plurality of layers of fibrous material.
This uniformity is accomplished by first superposing on a support,
layers of a fibrous material in sheet form, subsequently needling
each new layer onto the preceding ones as it is superposed thereon,
until the required thickness is reached, and modifying the distance
between the support and the needles, at each superposition of a new
layer, to keep a substantially constant needling depth, right
through the entire needling of the stack of layers.
The support and needles are mutually moved apart as each new layer
is superposed, by a distance equal to the thickness of a needled
layer. Thus, the needling is performed with a uniform density
throughout the whole thickness of the structure.
The process according to the invention is applicable both in the
manufacture of three-dimensional structures formed with superposed
flat sheets of fibrous material, and the manufacture of
three-dimensional structures of revolution, by needling of
superposed layers, formed by winding of a sheet of fibrous fabric
on a mandrel.
According to yet another feature of the invention, after the last
layer has been stacked and once needled, finishing needling steps
are carried out, with the distance between the support and the
needles being modified after each finishing needling step, so that
the needling density inside the upper layers is substantially equal
to that in the other layers.
Additionally, as a further feature of the invention, the support is
formed of a material that permits the needles to penetrate it so
that the initial layers of material are needled with the same
needle penetration as later layers so that their needling is of the
same characteristics as later layers.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more readily understood on reading the
following description with reference to the accompanying drawings
in which:
FIG. 1 is a diagrammatic view illustrating the process according to
the invention for the manufacturing of three-dimensional structures
by needling of flat layers;
FIGS. 2 to 4 are cross-sectional views of different stages in the
production of a structure with the process illustrated in FIG.
1;
FIG. 5 is a diagrammatic view illustrating the application of the
process according to the invention to the manufacture of
three-dimensional structures of revolution;
FIGS. 6 to 8 are cross-sectional views illustrating
diagrammatically the application of the process according to the
invention to the manufacture of three-dimensional structures of
revolution;
FIGS. 9 and 10 are diagrammatic sectional views illustrating other
embodiments of the process according to the invention for the
manufacturing of three-dimensional flat structures and of
three-dimensional structures of revolution, respectively; and
FIGS. 11 to 14 are diagrammatic views illustrating various forms of
fibrous material which can be used for carrying out the process
according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A first embodiment of the invention will be described with
reference to FIGS. 1 to 4.
On a horizontal platen 10 are brought one by one, sheets 20 of
fibrous material of width and length determined as a function of
the structure to be produced. Said sheets 20 are stacked one on top
of the other and bonded together by needling using a needle board
12. The board 12 is situated above the platen 10 and extends in
parallel to one of the sides of the platen 10, and over a length
substantially equal to that of the sides, the needles 13 being
directed vertically downwards.
The needle board 12 is integral with a driving device (not shown)
which, in manner known per se, imparts to the needles a vertical
reciprocating motion 12a.
The needle board 12 and the stack of sheets 20 are additionally
movable one with respect to the other, horizontally and vertically.
Horizontally, the platen 10 is, for example, reciprocally driven in
motion 12b with respect to a support table 14, perpendicularly to
reciprocal motion of the board 12, under the action of reciprocal
driving means 10a mounted on the table 14. Vertically, the platen
10 and the board 12 are driven apart in steps 12c, for example,
achieved by driving the table 14 relative to the needling board by
an elevator 14a with a motor 14b under control of a controller 14c
or an endless screw or other coupling means mounted on the support
frame of the needle board. Vertical control means are shown more
fully in the above-identified parent application incorporated
herein by reference.
The manufacture of a structure is carried out as follows:
A first sheet of material is placed on the platen 10, is needled,
and then a second sheet is placed over it and needled thereto, the
needle board 12 being moved according to reciprocal vertical
movement 12a whereas the platen is moved horizontally in motion 12b
over a length at least equal to that of the sheet 20, in order to
cause these to be needled over their entire length by the board 12.
When the platen 10 has reached one end of its stroke, a new sheet
20 is stacked over the others and the table 14 is lowered a step
distance 12c corresponding to the thickness "e" of a needled strip,
before another needling cycle is performed by moving the platen 10
to the other end of its stroke. The procedure is thus continued,
the platen 10 being supplied with a new sheet at each end of its
horizontal stroke, until the height required for the structure is
reached.
At every penetration of the needles, their barbs carry with them
some fibers from the material of each sheet which they penetrate,
the fibers thus carried by the barbs creating vertical cross-linked
bonds between the superposed layers.
FIGS. 2 and 3 show the needles respectively in high and low
position. The needles penetrate into the texture through a depth
equal to several times the thickness of a needled sheet 20 (such as
for example 8 times and typically less than the total number of
sheets to be stacked). The needling depth is kept constant
throughout the entire operation due to the progressive lowering of
the structure with respect to the needles. It is necessary, for
needling the first sheets on the platen 10, to provide means
preventing the needles 13 from abutting against the hard surface of
the platen 10. To this end, the platen 10 is provided with a top
layer 11 into which the needles can penetrate without any risk of
being damaged and without carrying back particles or fibers into
the structure to be produced.
Said layer 11 can, for example, comprise a sheet 11a of reinforced
elastomer (such as, for example, reinforced "Hypalon" or a "Nylon"
material) fixed to the platen 10 and on which is adhesively fixed a
layer 11b formed of a felt base (for example, a polypropylene felt)
o sufficient thickness for the needles to penetrate, during the
first needling strokes, through the provided needling depth without
touching the platen 10. On said felt base is adhesively fixed
another sheet 11c, for example of polyvinylchloride. During the
needling, the sheet 11c is traversed through by the needles but
inhibits fibers from the material of the sheets 20 being thrust
into a linking with the felt base which would complicate the
separation of the completed structure from the layer 11.
It will be noted, as a variant, that it is possible either to keep
the platen immobile horizontally and thus to move the needle board,
or to conduct the needling on a perforated platen with holes 11'
aligned with the needle position on each stroke as shown in FIG. 9.
In this case, the platen 10' need not be covered with a surface
layer such as layer 11. The platen 10 is immobile respect to the
needle board 12 in horizontal direction, therefore it is the stack
of sheets which is moved horizontally on the platen at every
needling stroke.
In order to obtain a constant needling density through the whole
thickness of the structure, different finishing needling cycles are
used after needling of the last sheet. As the supports are stepped
apart without the addition of a new layer, the needles then go
through an increasing distance "d" in the air before reaching the
structure and arriving to the end of their downstroke (FIG. 4). The
barbs are thus less clogged than if they had to go an equal
distance "d" through the fibrous fabric. The needles are thus
increasingly effective during the finishing needling strokes.
Therefore, to avoid denser needling in the upper layers, the number
of finishing operations or strokes carried out (four for example)
is less than (eight) which would have been used with the addition
of a new layer at each stroke.
The sheet of fibrous material may be supplied in different forms,
particularly depending on the proposed application.
For example, the fibrous material may be at least partly
constituted by a layer of discontinuous fibers obtained by carding
(card webs), or by a layer of continuous fibers obtained by
criss-crossing unidirectional webs of continuous yarns or tows
followed by low density needling (pre-needling) of the layer
together as shown in FIG. 11. In the latter case, the
criss-crossing can be achieved as already known per se, namely by
lapping. To achieve this, one of the unidirectional webs of yarns
or tows is continuously supplied whereas another unidirectional web
of yarns or tows is superposed thereon by reciprocal movement in a
direction perpendicular to the movement of the web. Due to the
relative displacement between the unidirectional webs, what is
obtained is in fact three superposed webs forming between them
angles different from 90.degree., for example angles of
60.degree..
When greater mechanical strength is required for the structure due
to the properties needed for the final composite fabric to be
produced, the fibrous material is constituted by at least one woven
layer, which is, for example:
a complex constituted by a fabric of continuous or discontinuous
fibers (satin or plain weave type) as shown in FIGS. 13 and 12,
respectively on which has been needled, with low needling density a
web of discontinuous fibers obtained by carding (card webs) or a
web of continuous fibers, such as a web being deposited over the
fabric by lapping, or
a fabric on its own, constituted by yarns in the warp direction,
said yarns being continuous or discontinuous, and by a roving in
the weft direction (FIG. 14).
The fibers constituting the materials described hereinabove may be
any natural, artificial or synthetic fibers, used either such as
they are or after heat treatment, the choice of the nature of these
fibers being dependent on the proposed application.
In the case of the manufacture of reinforcement structures for
composite materials destined to withstand great thermomechanical
forces, the most advantageous fibers are the carbon fibers and the
ceramic fibers (alumina, silicon carbide, etc.) as well as any
fiber precursors of such fibers, or any fibers constituting an
intermediary between such precursor fibers and the fully
heat-treated fibers.
When the three-dimensional structure is entirely or partly produced
from precursor or intermediate fibers, it has to undergo later a
heat-treatment to confer to the fibers the maximum mechanical
properties.
This last manner of proceeding protects the fibers against breaking
in the case where the heat-treated fibers have modules that are too
high and transversal strengths that are too low to be needled
without any damage, as this is the case with carbon and ceramic
fibers. Thus the materials described hereinabove can be constituted
at least partly with carbon and ceramic precursor fibers, any
remaining fibers being in carbon or ceramic.
For example, the fibrous material destined for needling is
constituted by a complex formed of a highly resistant fabric or
carbon fibers, pre-needled with a card web of stabilized P.A.N.
fibers (polyacrylonitryle, a carbon precursor). In this complex,
the fabric brings the required mechanical strength whereas the web
of fibers permits a non-destructive needling of the superposed
sheets, since the barbs of the needles, in loading themselves with
the stabilized P.A.N., cannot seriously damage the carbon fibers.
For economical reasons, the fabric will be selected to be as light
as possible in view of the required mechanical properties, for
example with a weight varying between 100 and 600 g/m2.
It is of course possible in the preceding example to replace carbon
fibers and/or precursors independently by ceramic fibers and/or
precursors. And, in reverse, a fabric in carbon or ceramic
precursor fibers may be combined with a card web in carbon or
ceramic fibers.
In the same way, carbon or ceramic fibers and precursor fibers of
carbon or ceramic can be combined and form, independently one from
the other, the warp and weft of a fabric constituted by yarns of
continuous or discontinuous filaments in the warp direction and by
a roving in the weft direction. A special example of manufacture of
a three-dimensional structure destined to the production of brake
discs in carbon-carbon composite material is given hereunder.
EXAMPLE 1
The production of sheets of fibrous material starts with tows of
polyacrylonitryle (P.A.N.) fibers such as those commercialized by
the British company COURTAULDS, under the denomination Courtelle
SP30, a tow being composed by 160,000 filaments. After
pre-oxidation, unidirectional webs are formed by placing a
plurality of tows one next to the other. A sheet of fibrous
material is obtained by spreading and/or lapping one web over
another in the manner indicated hereinabove, this giving three
unidirectional layers between which the rows are offset by
60.degree., then by a light needling, the sole object of which
being to bond the layers together sufficiently to allow the
handling of the sheet. Said sheet is stored on a roll.
The sheet is cut into pieces, the length and width of which
correspond to the length and width of the structure to be produced.
The pieces of sheet are then stacked onto a platen, being each time
needled to the preceding ones in the manner described hereinabove.
The number of needles carried by the needle board and the frequency
at which said board is actuated for a given horizontal speed of the
platen, are determined so as to obtain the required needling
density, which in the present example is about 48 strokes or needle
penetrations per cm2. The depth of penetration is determined as a
function of the number of layers to be traversed through by the
needles, for example about 20 layers. After each needling
operation, the platen is lowered with respect to the needles a
value equal to the thickness of one needled layer, which, in this
example, is about 0.8 mm. After the needling of the last deposited
sheet, a predetermined number of finishing needling cycles are
performed; in this case, this number is equal to 6 (three
return-strokes of the platen).
To produce a brake disc from such a structure, a ring is cut
therefrom and subjected to the normal operations of carbonization
(so as to transform the pre-oxidized P.A.N. into carbon) and of
densification by chemical vapor infiltration of pyrocarbon (CVI) in
several cycles until a density of about 1.75 is obtained.
A second application of the process according to the invention will
be described with reference to FIGS. 5 to 8.
A sheet 40 of fibrous material is wound in continuous manner on a
mandrel 30 driven in rotation about a horizontal axis. The mandrel,
which in this case is cylindrical with circular cross-section, has
a profile corresponding to that of the axi-symmetrical structure to
be produced, whereas the sheet 40 has a width corresponding
substantially to that of said structure. The sheet 40 is wound on
the mandrel to form superposed layers which are joined together by
needling, by means of a needle board 32. The needle board is
situated above the mandrel 30 and extends in parallel to the axis
of the latter over a length at least equal to the width of the
sheet 40. Needles 33 of the board 32 are directed vertically
downward.
In addition to the reciprocal vertical motion, the needle board 32
and the mandrel 30 are movable one with respect to the other in a
vertical direction. To this effect, the shaft of the mandrel passes
through bearings mounted in supports 34 movable vertically with
respect to the frame on which is mounted the needle board. The
vertical displacement of the supports 34 is produced by means of
stepper motors which are controlled synchronously to each other and
which drive toothed wheels on which mesh endless chains 30, the
support 34 being fixed on the latter. The mandrel 30 is
continuously driven in rotation by a motor 37 provided at one end
of the axis of the mandrel.
A constant needling density per unit of surface is achieved by
varying the rotational speed of the mandrel as a function of the
diameter of the structure being formed, the needling frequency
being constant. Of course, a similar result could be obtained by
varying the needling frequency (i.e. the frequency of needle
strikes), the rotational speed of the mandrel remaining
constant.
Each time another layer is formed by winding the sheet 40 on the
turning mandrel 30, the mandrel is lowered with respect to the
needle board by a distance corresponding to the thickness "e" of
the needled layer, and a new needling cycle is performed.
Simultaneously to the winding, the sheet 40 is needled onto the
preceding layers at the very location where the sheet is superposed
on, and tangent to the latter. At each penetration of the needles,
the asperities or barbs of the needles carry with them fibers from
the material of the layers traversed, these fibers creating radial
bonds between the superposed sheets.
FIGS. 6 and 7 show the needles respectively in the high position
and in the low position. The needles penetrate into the layers
through a depth equal to several times the thickness of a needled
layer (for example, eight times). Due to the progressive lowering
of the mandrel 30 with respect to the needles, the needling depth
is kept constant throughout the process.
In order to be able to needle the first layers on the mandrel 30,
it is necessary to provide means that prevent the needles 33 from
hitting the hard surface of the mandrel 30. To this effect, the
mandrel 30 is coated with a base layer 31 which the needles can
penetrate without being damaged and without carrying particles or
fibers into the structure to be produced. The coating 31 can for
example be constituted of the same elements as layer 11 described
hereinabove.
To prevent the formation of circumferential lines of needle strokes
resulting from consistent placement of each stroke which impair the
homogeneity of the resulting structure, a reciprocal displacement
of small amplitude in axial direction is made between the mandrel
30 and the needle board 32. This is achieved for example by
imparting to the needle board a horizontal to-and-fro movement over
a small distance.
It will be noted as a variant that it is possible to carry out the
needling on a perforated mandrel 30' (FIG. 10) having perforations
31' (FIG. 10) aligned to the placement of each stroke or hole of
the needles of the board 32. In this case, it is not necessary to
coat the mandrel with a layer 31. The mandrel is then immobile with
respect to the needle board except vertically and it is the sheet,
deposited on the mandrel, which is driven in order to be wound over
the mandrel. In a manner known per se, the driving of the sheet 40
may be caused by one or several members such as a cylinder acting
by friction on the outer surface of the structure being produced.
It will be further noted that this particular driving method is
applicable when using a mandrel with a layer 31, with the rotary
drive operating by friction on the outer surface of the structure
in place of driving the end of the axis of the mandrel by a
motor.
In order to obtain a constant needling density through the whole
thickness of the structure, it is necessary to conduct finishing
needling cycles once the last layer has been positioned and
needled. The procedure then is the same as described hereinabove
relative to the needling of flat sheets, taking into account the
fact that the needles then go through an increasing distance "d" in
the air before reaching the structure and arriving at the end of
their downward stroke (FIG. 8).
The fibrous material of the sheet is selected in particular as a
function of the proposed application, as described hereinabove
relatively to the material of the sheet 20.
A special example of manufacture of a cylindrical structure
destined to the production of brake discs in carbon-carbon is now
described.
EXAMPLE 2
The operation starts from tows of P.A.N. fibers as in Example 1.
After pre-oxidizing, one part of the tows is subjected to stretch
breaking, crimping and weaving operations in order to obtain a
fabric of about 450 g/m2, and the other part is subjected to
crimping, cutting and carding operations in order to obtain a card
web of about 120 g/m2. The aforementioned operations are
conventional operations in the textile industry. A sheet of fibrous
material is obtained by lapping and pre-needling of the card web
onto the fabric, the object of the pre-needling being simply to
give the sheet sufficient cohesion for subsequent handling. The
sheet so produced can then be stored on a roll.
In order to form the three-dimensional structure, the sheet is
wound on a mandrel while being needled to itself according to the
invention. At each rotation of the mandrel, it is moved away from
the needles a distance equal, in this case, to 0.9 mm, which is the
thickness of a needled layer. The needling density is selected to
be substantially equal to that indicated in Example 1. After the
positioning of the last layer, several finishing needling cycles
are conducted; these operations being carried out, in the
illustrated example, over six complete rotations of the mandrel.
The resulting cylindrical structures show a fiber density rate
varying between 43% and 46%.
To product brake discs, the axisymmetrical structure is cut
radially into rings which are thereafter subjected to the
conventional operations of carbonization and densification by the
CVI process as described above. The above-described system of
needling can be practiced in other embodiments within the scope of
the invention as defined solely in the following claims.
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